1. Field of the Invention
The invention lies in the field of internal combustion engines. More specifically, the invention relates to a method of regenerating a NOx (nitrogen oxide) storage catalyst which is arranged in the exhaust tract of an internal combustion engine that is operated with air excess. A NOx measurement transducer is arranged downstream of the storage catalyst. In a regeneration phase, the catalyst converts stored NOx catalytically, with the addition of a reducing agent, the reducing agent being produced by the internal combustion engine being operated briefly with a rich air/fuel mixture (i.e., lambda less than 1).
In order to reduce the fuel consumption of gasoline internal combustion engines further, internal combustion engines with lean combustion are used with increasing frequency. In order to fulfil the exhaust-gas emission limit values required, special exhaust-gas aftertreatment is necessary in such internal combustion engines. NOx storage catalysts are used for this purpose. These NOx storage catalysts, because of their coating, are capable, during a storage phase, of absorbing from the exhaust gas NOx compounds which occur in the case of lean combustion. During a regeneration phase, the absorbed or stored NOx compounds are converted into harmless compounds by the addition of a reducing agent. The reducing agent used for lean-burn gasoline internal combustion engines may be CO, H2 and HC (hydrocarbons). These are produced by the internal combustion engine being operated briefly with a rich mixture and are made available to the NOx storage catalyst as exhaust-gas components, with the result that the stored NOx compounds in the catalyst are broken down.
Efficiency of such an NOx storage catalyst depends essentially on optimum regeneration. If the quantity of regeneration agent is too low, the stored NOx is not sufficiently broken down, this being detrimental to the efficiency with which NOx is absorbed from the exhaust gas. If the quantity of regeneration agent is too high, although optimum NOx conversion rates are achieved, there is nevertheless an inadmissibly high emission of reducing agent. The optimum quantity of regeneration agent fluctuates over the service life of a vehicle. The possible cause of this may be seen in the change in the NOx mass flow emitted by the internal combustion engine. Another reason is the change in storage capacity of the catalyst, which decreases, for example, due to the storage of sulfate, since sulfur present in the fuel is burnt to form SO2, is oxidized into sulfate by the catalyst with air excess and is stored by the coating in a similar way to NO2. However, the binding of sulfate in the store is substantially greater. During a regeneration phase, however, sulfate is not converted, but remains bound in the NOx storage catalyst. With increasing sulfate storage, therefore, the capacity of the NOx storage catalyst decreases.
A commonly assigned German patent application DE 197 05 335 describes a method for triggering a sulfate regeneration for an NOx storage catalyst, in which a sulfate regeneration phase is carried out at predetermined points in time. When sulfate regeneration is triggered, not only the quantity of stored sulfate, but also the thermal aging of the NOx storage catalyst is taken into account.
European published patent application EP 0 597 106 A1 discloses a method for regenerating an NOx storage catalyst, in which the quantity of NOx compounds which is absorbed by the NOx storage catalyst is calculated as a function of operating data of the internal combustion engine. When a predetermined limit quantity of NOx stored in the NOx storage catalyst is exceeded, a regeneration phase is initiated. This does not, however, ensure that the exhaust-gas emission limit values are adhered to reliably.
In order to check the NOx storage catalyst, an NOx measurement transducer is normally arranged downstream of the catalyst. Such a measurement transducer is known, for example, from N. Kato et al., xe2x80x9cPerformance of thick film NOx sensor on diesel and gasoline enginesxe2x80x9d, Society of Automotive Engineers, Publication No. 970858.
The object of the present invention is to provide a method of regenerating a NOx storage catalyst which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and in which the regeneration of an NOx storage catalyst is carried out in such a way that the latter is operated with optimum efficiency.
With the above and other objects in view there is provided, in accordance with the invention, a method of regenerating a NOx storage catalyst of the type arranged in an exhaust tract of an internal combustion engine operated with air excess, wherein a NOx measurement transducer is disposed downstream of the Nox storage catalyst in an exhaust gas flow direction, the method which comprises:
during a regeneration phase, briefly operating the internal combustion engine with a rich air/fuel mixture to produce a reducing agent and catalytically converting NOx stored in the Nox storage catalyst with the addition of the reducing agent;
providing an amperometric NOx measurement transducer formed with a solid-state electrolyte and having a first measuring cell, a second measuring cell, a reference electrode exposed to ambient air, a first electrode, a second electrode, and an outer electrode;
measuring an oxygen concentration in the first measuring cell via a first Nernst voltage between the first electrode and the reference electrode and regulating the oxygen concentration with a first oxygen-ion pumping current between the first electrode and the outer electrode;
measuring an oxygen concentration in the second measuring cell via a second Nernst voltage between the second electrode and the reference electrode; and
with the first and second measuring cells connected in series, picking off a voltage between the outer electrode and the reference electrode, and detecting an output signal dependent on the oxygen concentration and exhibiting two-position behavior during the regeneration phase; and
deriving from a time profile of the output signal a criterion as to whether the quantity of regeneration agent must be changed in order to achieve optimum regeneration of the NOx storage catalyst.
In other words, in the regeneration phase, a signal picked off at an NOx measurement transducer is evaluated in order to establish whether the quantity of regeneration agent was optimum. The signal used for this purpose is picked off at an amperometric NOx measure transducer. This signal reproduces the lambda value or the oxygen concentration in the exhaust gas and has two-position behavior, that is to say, in the range of lambda=1, the signal changes sharply in the case of low lambda changes.
In accordance with an added feature of the invention, two sum values are formed as criterion, with:
a first sum value being calculated from the output signal scanned at a specific frequency, from a start of regeneration until a predetermined threshold value is exceeded;
a second sum value being calculated from the output signal scanned at the specific frequency, from a point when the threshold value is exceeded until the sum value falls below the threshold value;
comparing the sum values with associated threshold values; and
determining the quantity of regeneration agent in dependence on a result of the comparison.
In accordance with an additional feature of the invention, the determining step comprises selectively keeping the quantity of regeneration agent constant, increasing the quantity of regeneration agent, or reducing the quantity of regeneration agent in dependence on the result of the comparison.
In accordance with another feature of the invention, the quantity of regeneration agent is kept constant when the first sum value is higher than a threshold value and the second sum value is within a range delimited by a lower threshold value and an upper threshold value.
In accordance with yet another feature of the invention, the quantity of regeneration agent is increased when the first sum value is higher than a threshold value and the second sum value is lower than a lower threshold value.
In accordance with again another feature of the invention, the quantity of regeneration agent is reduced when the first sum value is higher than a threshold value and the second sum value is higher than an upper threshold value.
In accordance with a further feature of the invention, the quantity of regeneration agent is increased by extending the regeneration phase.
In accordance with again a further feature of the invention, the quantity of regeneration agent is reduced by shortening the regeneration phase.
In accordance with yet an added feature of the invention, the method comprises shortening a duration of a storage phase of the NOx storage catalyst, during which the internal combustion engine is operated with air excess, and carrying out sulfate regeneration for the storage catalyst when the sum value is lower than the threshold value.
In accordance with yet an additional feature of the invention, the output signal is corrected as a function of the first oxygen-ion pumping current, to compensate for an error voltage stemming from a transition resistance through which the first oxygen-ion pumping current flows.
In accordance with a concomitant feature of the invention, the output signal is corrected in dependence on the temperature of the measurement transducer.
In a preferred embodiment of the method, the quantity of regeneration agent to be supplied to the NOx storage catalyst is matched to the optimum value. Since a greatly reduced reducing agent requirement stems from a fallen storage capacity of the NOx storage catalyst, preferably sulfate regeneration can be carried out when the storage capacity has fallen too sharply.
The advantage which can be achieved by means of the invention is therefore, in particular, that the optimum quantity of regeneration agent is supplied over the entire service life of the vehicle.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for regenerating an NOx storage catalyst, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.